1. Field of the Invention
The present invention relates to a charged particle beam exposure method for employing a charged particle beam, such as an electron beam or an ion beam, when performing an exposure, and in particular, to a charged particle beam exposure method, and an apparatus therefor, for correcting for the shifting of an exposure position, which accompanies the thermal expansion of a wafer, or beam drift due to a charge-up, which occurs when the entire surface of a wafer is exposed within a short period of time.
2. Related Arts
An electron beam exposure method (hereinafter this phrase is employed to simplify the explanation), one of the charged particle beam exposure methods, is a lithography tool that has both a high resolution capacity and a pattern generation capacity, and that is employed for the exposure of a reticle or the direct exposure of a semiconductor wafer. However, with the common electron beam exposure method, a desired drawing pattern is calculated, based on the electronic data for a pattern to be drawn, and an electron beam for the pattern is sequentially emitted to expose a reticle or a wafer. Since an area that can be exposed by a single emission beam is comparatively narrow, a process for using an electron beam having a predetermined pattern to expose a plurality of fields obtained by dividing a chip must be repeated a number of times. As a result, a long time is required for an exposure, while the throughput is smaller than that provided by a stepper exposure method that uses light.
At present, therefore, the electron beam exposure method is mainly used with a procedure employed for exposing a reticle, or with a procedure for exposing a sample for a new device. Thereafter, a reticle mask produced in this manner is employed to expose a large area on a wafer by employing the stepper exposure method that uses light, so that semiconductor devices can be mass produced.
A high density and microstructures will be required in the future for large capacity semiconductor devices, such as DRAMs (Dynamic Random Access Memories), and it is anticipated that the conventional stepper exposure method will not be able to cope with the revolution increase.
In order to fabricate 256M bit DRAMs, because of microstructure, the employment of the technique for directly exposing a wafer by using the electron beam exposure method is inevitable. For this reason, the conventional problem posed by inadequate throughput must be resolved, and the exposure accuracy must also be improved, even as the throughput is increased.
Assuming that the technique for exposing a wafer directly to an electron beam is the technique employed for the mass production of semiconductor devices, in this case, it is necessary to draw a single 8-inch wafer about five minutes, for example. To effect an exposure in such a short time, a block exposure method or a blanking aperture array method, both of which have been conventionally proposed, could be employed. According to these methods, a general pattern obtained by combining several variable rectangular beam shots can be exposed within a single shot, and therefore a throughput is expected to be increased several times. However, the number of electron beams per unit time with which the wafer will be irradiated will also be increased several times. The accumulated quantity of charged particles is referred to as the "volume" herein.
A limit is imposed on the current of an electron beam when an image is blurred by Coulomb interactions between electrons. For example, 2 to 4 .mu.A is the limit for an image dissection of 0.2 .mu.m. When a beam current exceeds the limit, an irradiated image will be blurred by the interaction of the electrons in the beam. Therefore, for resolution, a beam current of about 0.2 .mu.m is considered to be the limit. With an acceleration voltage of 50 kV, a wafer is irradiated with an energy of 0.1 to 0.2 W, and for five minutes (300 seconds) a thermal energy of 30 to 60 J (Joule) is supplied to the wafer. Even assuming that 50% of this thermal energy is transferred from the wafer to the holder of the apparatus and exhausted, if the thermal capacity of the 8-inch wafer is approximately 16 J/K, the temperature of the wafer is raised 1 to 2.degree. C. When the linear expansion of a Si wafer is 2.6 ppm/K, a thermal expansion with a coefficient of 3 to 5 ppm (point per meter: .mu.m to be expanded per 1 m) will occur.
For the 8-inch wafer, for example, the actual shift caused by the thermal expansion is 0.45 to 0.75 .mu.m with an expansion coefficient of 3 to 5 p.mu.m. Since the shifting distance from the center is about 0.3 .mu.m, which is about the half of the above value, this shifting can not be ignored when fabricating 256-M DRAM, for which a line width of 0.2 .mu.m is required.
To improve the throughput when a wafer is directly irradiated by an electron beam, an effective method is the continuous movement method, one whereby a stage on which the wafer is mounted is in constant movement while the wafer is being exposed. In a conventional step-and-repeat method in which an exposure in a main deflection area is repeated by moving a stage between the areas, the setting time accompanied by a halt following the movement of the stage prevents the improvement of the throughput.
According to the continuous wafer movement method, a wafer travels along a frame area that is narrower than a single chip but is sufficiently wide to permit a main deflector, constituted by an electromagnetic coil, to deflect the electron beam. Therefore, a single chip area is covered with a plurality of frame areas, and even if the location of a positioning mark, which is provided near a chip area on a wafer, is detected and the origin of the beam is corrected, it takes an extended period of time for the exposure of the pertinent chip area to be completed. Since irradiation of a large volume of an electron beam is performed during this period, the temperature of the wafer is raised and the wafer is expanded. This problem can not be avoided when the continuous wafer movement method, together with a stronger electron beam, is employed to perform direct exposure of a wafer for fabrication of a microstructure.
Further, the increase in the intensity of the beam is accompanied not only by a rise in the temperature but also by beam drift, which is caused by a charge-up related to the contamination of the exposure apparatus. The accumulation of electric charges in an organic substance, such as a resist attached to an electrostatic deflector, that results in a shift in the deflection distance of the deflector is known as beam drift, which is due to a charge-up.
The explication of the behavior of beam drift is very difficult, and how a value to compensate for beam drift should be determined has not yet been resolved. It is proposed that, when the intensity of a beam is increased for direct irradiation of a wafer, the drift of the beam will be changed during the irradiation of a single wafer. This problem must also be resolved.